I posted a few times already about Allen et al’s paper on the Trillionth Tonne, ever since I saw Chris Jones present it at the EGU meeting in April. Basically, the work gets to the heart of the global challenge. If we want to hold temperatures below a 2°C rise, the key factor is not how much we burn in fossil fuels each year, but the cumulative emissions over centuries (because once we release carbon molecules from being buried under the ground, they tend to stay in the carbon cycle for centuries).
Allen et. al. did a probablistic analysis, and found that cumulative emissions of about 1 trillion tonnes of carbon give us a most likely peak temperature rise of 2ºC (with a 90% confidence interval of 1.3 – 3.9°C). We’ve burnt about half of this total since the beginning of the industrial revolution, so basically, we mustn’t burn more than another 1/2 trillion tonnes. We’ll burn through that in less than 30 years at current emissions growth rates. Clearly, we can’t keep burning fossil fuels at the current rate and then just stop on a dime when we get to a trillion tonnes. We have to follow a reduction curve that gets us reducing emissions steadily over the next 50-60 years, until we get to zero net emissions. (One implication of this analysis is that a large amount of existing oil and coal reserves have to stay buried in the ground, which will be hard to ensure given how much money there is to be made in digging it up and selling it).
Anyway, there’s now a website with a set of counters to show how well we’re doing: Trillionthtonne.org. Er, not so well right now, actually.